Nutrient runoff dynamics in a rural catchment: Influence of land-use changes, climatic fluctuations and ecotechnological measures

Ecological Engineering 14 (2000) 405 – 417

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Nutrient runoff dynamics in a rural catchment: Influence of land-use changes, climatic fluctuations and ecotechnological measures U8 lo Mander a,*, Ain Kull a, Valdo Kuusemets a, Toomas Tamm b b

a Institute of Geography, Uni6ersity of Tartu, 46 Vanemuise St., EE-2400 Tartu, Estonia Institute of Water Management, Estonian Agricultural Uni6ersity, 5 Kreutzwaldi St., EE-2400 Tartu, Estonia

Abstract The main trend in land-use changes in the Porijo˜gi River catchment, south Estonia, is a significant increase in abandoned lands (from 1.7% in 1987 to 10.5% in 1997), and a decrease in arable lands (from 41.8 to 23.9%). Significant climatic fluctuations occurred during the last decades. Milder winters (increase of air temperature in February from −7.9 to −5.5°C during the period 1950 – 1997) and a change in the precipitation pattern have influenced the mean annual water discharge. This results in more intensive material flow during colder seasons and decreased water runoff in summer. During the period 1987 – 1997 the runoff of total-N, total-P, SO4, and organic material (after BOD5) decreased from 25.9 to 5.1, from 0.32 to 0.13, from 78 to 48, and from 7.4 to 3.5 kg ha − 1 year − 1, respectively. Most significant was a 4–20-fold decrease in agricultural subcatchments while in the forested upper-course catchment the changes were insignificant. Variations of total-N, and total-P runoff in both the entire catchment and its agricultural subcatchments are well described by the change of land use (including fertilization intensity), soil parameters and water discharge. In small agricultural subcatchments the rate of fertilization was found to be the most important factor affecting nitrogen runoff, while land-use pattern plays the main role in larger mosaic catchments. Ecotechnological measures (e.g. riparian buffer zones and buffer strips, constructed wetlands) to control nutrient flows from agricultural catchments are very important. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Land-use change; Climatic fluctuations; Nutrient losses; Ecotechnological measures

1. Introduction Although nutrient losses from rural catchments have been well studied (Schreiber and Neumaier, * Corresponding author. Tel: + 372-7-375819; fax: + 372-7375825. E-mail address: [email protected] (U8 . Mander)

1987; Fleischer and Hamrin, 1988; Kelly, 1988; Lowrance and Leonard, 1988; Kronvang et al., 1993), few investigations have analyzed long-term trends of nutrient cycling (Jordan et al., 1986). Previous analyses of the impacts of different human activities on nutrient losses from rural catchments include: general land-use intensification (White et al., 1981; Cameron and Wild, 1984),

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fertilization (Miller, 1979), afforestation (Likens and Bormann, 1995), and stream channelization (Krug, 1993; Yarbro et al., 1984). However, less research has analyzed the importance of environmental improvement, such as extensification and reforestation. The impact of freshwater wetlands (Mitsch and Gosselink, 1986; Whigham et al., 1988) due to higher denitrification values (Jansson et al., 1994) has also been researched. In the long-term perspective of material cycling and energy flows, both land-use changes and climatic fluctuations play an essential role. Most early studies have dealt with land-use pattern influences on nutrient budgets, whereas climatic change analysis is a relatively new approach (Panagoulia, 1991; Arnell, 1992). As changing land use has a major influence on global climatic processes, and nutrient runoff is directly related to weather systems, these factors should be analyzed in combination (Unsworth and Wolfe, 1995). Significant changes in nutrient fluxes occur in Europe, especially in central and eastern European countries, where the collapse of collectivized agriculture has caused severe ecological and socio-economic consequences, including the change in both material and energy flows in rural landscapes. Since 1990, the application of commercial fertilizers in Estonia has dropped at an unprecedented rate and the application of manure has also decreased significantly (Mander and Palang, 1994). The same trend has been documented in other eastern European countries (Ola´h and Ola´h, 1996; Grimvall et al., 1999). In the long run, such dramatic changes in agricultural practices will inevitably influence the losses of nutrients to water, and studies of small catchments in eastern Europe have shown that the output of nitrogen and phosphorus with water responds rapidly to decreased fertilization (see Grimvall et al., 1999). Due to continuing reorganization of agriculture, however, some catchments may experience intensive agricultural activity. In that case the question is when and to what extent the increase in agricultural activities will reflect in nutrient flows. Therefore, various estimations and forecasts for further development of landscape functioning are very important (Mander and

Jongman, 1998). We suggest that some ecotechnological measures can play a vital role in controlling nutrient losses from rural catchments. In particular, riparian buffer zones (Haycock et al., 1997), riparian wetlands (Mitsch, 1992), and constructed wetlands for treatment of polluted waters from agricultural fields (Vought and Lacoursie`re, 1998) provide great interest. The main objectives of this study are: (1) to analyze the 10-year trends of nutrient and organic material runoff from a rural catchment in southern Estonia, (2) to clarify the impact of significant land use changes and variation of climatic parameters on nutrient runoff from the catchment, and (3) to analyze the potential effect of planned ecotechnological measures to regulate nutrient flows in this catchment.

2. Material and methods

2.1. Study area The Porijo˜gi River watershed, a part of the Lake Peipsi drainage basin, represents a typical south Estonian landscape. The central and northern parts of the catchment lie within the south east Estonian moraine plain 5–10 km south of Tartu (58°23%N; 26°44%E). The elevation of the moraine plateau is from 30 to 60 m above a.s.l. with undulated relief (slopes are normally 5–6%), and the landscape is dissected by primeval valleys (Varep, 1964). The southern part of the drainage basin lies on the northern slope of the Otepa¨a¨ Heights composed of moraine hills and kames with a great variety of glacial deposits. The elevation of this region is up to 120 m; the relative heights reach 30–35 m. About 50% of the moraine plain (mostly podzoluvisols, planosols, and podzols on loamy sands and fine sandy loams) is potential arable land. Gleysols and peatlands, partly used as perennial grasslands, dominate in valleys and other depressions. The northern part of the catchment is characterized by larger patches of fields, grasslands, and forests, while in the hilly southern section a mosaic moraine–hilly landscape dominates. In this area agricultural nonpoint pollution (e.g. fertilization

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and manure management) is a dominant pollution source. Point pollution sources (towns, factories) are insignificant. A detailed description of the study area is presented in earlier papers (Mander et al., 1989, 1995).

2.2. Field and laboratory studies Water discharge has been measured and water samples have been taken once a month since 1987 from the closing weirs of eight subcatchments of the Porijo˜gi River. The samples were analyzed for BOD, NH4-N, NO2-N, NO3-N, total-N, PO4-P, total-P, and SO4. All water analyses were made following the international methods for examination of water and wastewater quality (APHA, 1981). Three subcatchments with similar size but different land-use patterns were chosen for the detailed analysis. These are: (1) the forested upper course, (2) the Sipe ditch subcatchment, with intensive agriculture and seminatural riparian buffer zones, and (3) the Va¨nda ditch, with intensive agriculture but without significant measures to control nonpoint pollution (see Mander et al., 1996). In addition, the entire catchment of the Porijo˜gi River (258 km2 behind the Reola measuring point of the Estonian Meteorology and Hydrology Institute (EMHI) as closing weir) was used for detail analyses. Daily mean stream discharge (m3 s − 1) was determined beginning in 1985 at the Reola hydrological measuring point (EMHI), using the Parshall flume (Wanielista, 1990). The mean monthly stream discharge of Porijo˜gi River for the period 1951 –1985 is calculated by a regression formula (Mander et al., 1998). Several other regression formulae were used to calculate daily water discharges for three subcatchments (Mander et al., 1996). Runoff of organic matter (on the basis of BOD5), N, P, and S was calculated for each day (kg ha − 1 day − 1) using the flow-weighted values and then calculated as monthly and annual mean values (see Rekolainen et al., 1991). Potential evapotranspiration was calculated after the Monteith method with modifications by Allen et al. (1989). Calculations of actual evapotranspiration were performed according to the Oldekop method

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with modifications by Tamm (1994). The detailed hydrological data set of the whole study period is given in earlier papers (Mander et al., 1996, 1998). Land use was analyzed using 1:10 000 topographic maps from 1987 and upgraded cadastrial maps (1:20 000). The actual pattern of each year, especially the location of abandoned lands, was investigated in field trips. The following pattern was analyzed: (1) arable lands, (2) all grasslands and fallow lands, and (3) forests and wetlands.

3. Results and discussion

3.1. Land use changes in the Porijo˜gi Ri6er catchment During the study period, a significant change in the land-use pattern of the entire catchment in Porijo˜gi River was observed. Table 1 shows the variation of land-use types grouped into main classes and divided between the subcatchments. The entire catchment has been subject to an extensification of land use. For instance, the portion of abandoned lands and grasslands has increased from 14.8% in 1987 to 27.6% in 1997, with a maximum (29.5%) in 1995. At the same time arable lands have decreased from 41.8 to 23.9%. About 10.5% of former agricultural lands have been classified as fallow lands, portions of which are overgrown by underbrush. Forested areas together with wetlands have increased from 43.4 to 48.5%. A slight increase in wetlands (from 3.4 to 3.7%) is mainly due to deterioration of drainage systems. Subcatchments show notably different land-use changes. The wooded upper-course subcatchment experienced no significant change, whereas the Sipe and Va¨nda subcatchments showed a transition quite similar to the entire catchment. In the Sipe subcatchment, the arable land decreased from 58.5 to 19.1% and abandoned land together with grasslands increased from 6.9 to 38.5%. Forested areas and wetlands showed a slight increase. In the Va¨nda subcatchment, about 90% of the arable land became seminatural and cultivated grasslands (growth from 0.5 to 49.5% and from 0.9 to 24.6%, respectively). In both the entire

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catchment as well as in Sipe and Va¨nda subcatchments, the greatest depression of agricultural activities was in 1994 – 1995 (Table 1). A slight increase in arable land was observed in 1996 – 1997. Significant and rapid change in land-use intensity is the main factor in the reduction of the stream discharge and nutrient losses. This also caused changes in microclimate, especially by the changing evapotranspiration rate and soil water storage capacity.

tion of snow cover. Over the last decade snow cover duration has been about 40 days less than the longer period average (1920–1995). Until the l990s the average snow cover lasted 120 days while in the following years snow cover duration dropped to 80 days per year (Jaagus, 1997). Sleet, wet snow, and rain have replaced snow, resulting in quicker runoff, especially surface flow over the partly frozen surface. A reduced amount of accumulated snow in winter has led to lower peak flows in spring. The amount of precipitation has increased in autumn and winter and decreased significantly in summer. The annual precipitation patterns have also changed. In the 1950s precipitation was high (600 mm) followed by a dry period during the second half of the 1960s and 1970s (400 mm). The highest precipitation during the study period was 804 mm year − 1, characteristic of the very high average for the 1980s (average 670 mm). During the last decade annual precipitation has decreased, but remains higher than the longer period average (Fig. 2). Climatic changes directly influence hydrologic parameters, biogeochemical processes, and material flows. Milder winters result in shorter periods of snow cover and frozen surface, which gives rise

3.2. Variation of climate and ri6er discharge During the last decades considerable variation in climatic parameters has been observed, with a trend to a milder climate with more intensive cyclonic activity. The main changes in weather are related to the cold season. Due mainly to significantly warmer winters, the mean annual air temperature has increased from 4.1 to 5.3°C. During the last 30 years the mean monthly air temperature in February, usually the coldest month in Estonia, increased from − 7.9 to − 5.8°C (Fig. 1). This higher mean temperature in winter is accompanied by higher precipitation during the cold season, which decreases the duraTable 1 Land use change in the Porijo˜gi River catchment in 1987–1997 (%) Land use type

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

Porijo˜gi entire catchment (total area 258 km 2) Arable land 41.8 41.6 Grasslands and fallow lands 14.8 14.8 Forests and wetlands 43.4 43.6

40.8 15.2 44

34.4 20.5 45.1

30.1 23.1 46.8

27.7 25.4 46.9

25.7 26.9 47.4

24.5 27.8 47.7

22.5 29.3 48.2

23.2 28.5 48.3

23.9 27.6 48.5

Porijo˜gi upper course (12.3 km 2) Arable land Grasslands and fallow lands Forests and wetlands

6.4 14.1 79.5

6.4 14.2 79.4

6.5 14.3 79.2

6.4 14.7 78.9

6.4 15.3 78.3

6.5 15.6 77.9

6.5 15.9 77.6

6.7 16.3 77

6.9 16.4 76.7

6.5 16.6 76.9

6.2 16.8 77

Sipe ditch (9.0 km 2) Arable land Grasslands and fallow lands Forests and wetlands

58.5 6.9 34.6

56 9.3 34.7

53.7 11.2 35.1

44 19.8 36.2

36.4 26.2 37.4

27.6 33.6 38.8

21.2 39 39.8

18.2 40.5 41.3

14.7 43 42.3

18.2 39.4 42.4

19.1 38.5 42.4

Va¨nda ditch (2.2 km 2) Arable land Grasslands and fallow lands Forests and wetlands

68.1 2 29.9

55.9 14.2 29.9

54.9 15.2 29.9

52.6 17.3 30.1

56 13.7 30.3

24.5 45.2 30.3

20.2 49.2 30.6

11.2 58.1 30.7

12.1 57.2 30.7

18.6 50.6 30.8

24.6 44.6 30.8

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Fig. 1. Polynomial (3rd power) trendlines of the average monthly air temperature (A) and precipitation (B) in the Porijo˜gi River catchment. I – XII months.

Fig. 2. Long term annual variation and polynomial trendlines of water budget in the Porijo˜gi River catchment (mm): P, precipitation; ETa, actual evapotranspiration; Q, runoff; and d, soil water storage (d = P−Q −ETa).

to higher runoff in winter when ecosystem nutrient removal is low. Less snow cover also results in less frequent peak flows in spring. Spring peak flow in the Porijo˜gi River has dropped from 20 – 24 m3 s − 1 in the 1980s to 10 – 12 m3 s − 1 in the l990s. In the smaller Sipe and Va¨nda subcatchments the decrease in peak flow over last two decades is more pronounced and differences in flow vary up to four times. Groundwater recharge in cold periods is hindered due to frozen soil layer, resulting in low water discharge during the warm season. As the result of higher air temperature and precipitation during the last decade, both potential and actual evapotranspiration (PET and

ET, respectively) have increased, especially in early spring and the summer. ET has increased from the lowest level (400 mm year − 1) in the 1960s to 450 mm year − 1 in the 1990s. In several years high calculated values of PET do not reflect ET values because of low precipitation in the warm season. However, changes in land-use also result in higher ET values. In the Sipe subcatchment especially, the rapid decrease in stream discharge can be explained by higher actual evapotranspiration values. The long period runoff dynamics in the Porijo˜gi River catchment are dependent on precipitation (R 2 = 0.36). In the 1950s, when mean annual pre-

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cipitation was relatively high the mean annual runoff was about 200 mm. Several very dry years from 1963–1965 and subsequent amelioration resulted in very low runoff (110 mm) from 1963 – 1977. Rainy periods until the l990s were characterized by mean annual runoff of about 200 – 300 mm despite increasing evapotranspiration. However in the l990s, when precipitation dropped, the runoff decreased sharply. Subcatchment runoff dynamics analyzed since 1987 also show clear differences (Fig. 3). The mean annual stream discharge in the wooded subcatchment (Porijo˜gi upper course) varied between 0.09 and 0.14 m3 s − 1 with no significant decrease. However, as seen in the Fig. 3, the mean monthly water discharge showed rather big variations; from 0.005 to 0.30 m3 s − 1. The mean annual runoff in the Sipe and Va¨nda subcatchments decreased with a clear trend from 0.043 to 0.01 m3 s − 1 and from 0.038 to 0.018 m3 s − 1, respectively. Rapid extensification of land use was likely a significant factor that led to higher ET. Three possible mechanisms can explain this phenomenon. First, abandoned grasslands have a higher ET, especially those covered with underbrush. Likewise, uncut grassland on peatland soils

evaporate significantly more than managed ones (Mundel and Wellenbrok, 1978). Second, deep peat deposits along the Sipe stream are an important buffer between precipitation and water discharge. Third, some decrease in ET might occur during milder winters. In recent models for the calculation of actual evapotranspiration this is not taken into account (Tamm, 1994). To study these mechanisms in detail, a case study on water balance and quality measurements has been carried out recently in the upper course of the Sipe stream.

3.3. Dynamics of nutrient losses in subcatchments Runoff of organic matter and nutrients is affected largely by both land-use changes and climatic fluctuations. During 1987–1997 the material transport decreased from both the entire catchment and agriculturally used subcatchments, while no significant change was found in the wooded upper course (Fig. 4). Nitrogen runoff is characterized using the total inorganic nitrogen (TIN; sum of NH4-N, NO2-N and NO3-N) values, because total nitrogen values are missing for some water samples. However,

Fig. 3. Variation and linear trendlines of mean monthly water discharge (m3 s − 1) in the entire Porijo˜gi River catchment (A), Porijo˜gi upper course (B), Sipe (C), and Va¨nda subcatchment (D) in 1987 – 1997.

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Fig. 4. Change of arable land (%) and variation of organic matter (BOD5), total inorganic nitrogen (TIN; sum of NH4-N, NO2-N and NO3-N), NH4-N, and total-P runoff (kg ha − 1 year − 1) in subcatchments in the Porijo˜gi River basin in 1987 – 1997.

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most of the dissolved nitrogen in the river and its tributaries is well mineralized and the correlation between the total-N and TIN is very high (R2 = 0.944; total-N=0.076 +(1.143TIN). In the entire catchment the mean annual runoff of TIN decreased significantly; from 17.8 to 4.0 kg N ha − 1 year − 1 (Fig. 4). During the last 3 years N losses have been similar to those calculated in forested areas. In the Sipe subcatchment total-N runoff decreased from 3.8 to 0.6 kg N ha − 1 year − 1, in contrast to the Va¨nda subcatchment (from 19.8 to 3.0, with 57.0 kg N ha − 1 year − 1 in 1991). Very high nitrogen losses from the Va¨nda subcatchment are due to intensive use of mineral fertilizers and manure application in 1987 –1991 (Fig. 4). However, these values are still lower than documented in other papers on rural catchments (Miller, 1979; Kronvang et al., 1993). The ammonia nitrogen seems to follow a different pattern, which is related the land use changes. Although there is a decrease in NH4-N runoff in both the whole drainage basin and all subcatchments studied, the absolute values of NH4-N losses do not reflect the percentage of arable lands in the subcatchments. For instance, the upper course and the entire catchment have very different percentage of arable lands with a similar NH4-N runoff pattern (Table 1; Fig. 4). This is due to relatively high organic matter in forest soils of the upper course. Likewise, from the Va¨nda subcatchment with a wetland forest in the upper part, the NH4-N losses are high. Variation of average annual total-P losses was not as significant as total-N, although it indicated a decreasing tendency. The greatest decrease was in the cultivated Sipe and Va¨nda subcatchments where total-P runoff dropped from 0.32 to 0.02 and from 1.84 to 0.09 kg P ha − 1 year − 1, respectively. The rapid decrease in mean annual runoff was mainly due to decreasing stream discharge. In the forested upper course the average annual sulfate transport varied from 17 to 80 kg SO4 ha − 1 year − 1 with no clear trend. Likewise, there was no significant trend in SO4 transport in the whole drainage basin. Cultivated subcatchments of Sipe and Va¨nda experienced the greatest decrease with SO4 runoff dropping from 66 to 8 and from 153 to 33 kg SO4 ha − 1 year − 1, respectively. In addition,

a decrease in sulfate runoff suggests that the impact of acid deposition is not very high in this region. Mean annual losses of organic matter from the entire catchment are quite low and stable at 3.5– 7.4 kg BOD5 ha − 1 year − 1. The highest loss was recorded in the Va¨nda subcatchment, where pig slurry was applied in the fields. Organic matter loss in this area reduced from 24 to 4 kg BOD5 ha − 1 year − 1. Table 2 represents the linear correlation and regression equations between the dynamics of share of arable land and the nutrient losses in subcatchments. In the forested upper-course subcatchment the variation in agricultural lands has been insignificant (Table 1) and therefore does not play an essential role in the dynamics of nutrient fluxes. Conversely in Sipe and Va¨nda agricultural subcatchments the land-use change strongly correlates with losses of organic matter (based on BOD5; R 2 value varies from 0.55 to 0.42, respectively), total inorganic nitrogen (TIN; sum of NH4-N, NO2-N and NO3-N; R 2 = 0.74 and 0.56, correspondingly), total phosphorus (R 2 = 0.83 and 0.71) and sulfates (R 2 = 0.83 and 0.55, respectively). In the entire catchment the change of arable land significantly correlates with losses of organic matter (BOD5), TIN and total-P (R 2 = 0.90, 0.66 and 0.88, correspondingly). Stream discharge and nutrient/organic matter runoff from the entire catchment and subcatchments correlated positively (for BOD5 and SO4 PB 0.01), but there was no significant correlation between stream discharge and total nitrogen runoff. Phosphorus runoff has always been reported to correlate positively with stream runoff. As in our study area, the relationship between nitrogen and stream discharge depends on catchment characteristics. Weather conditions also affect seasonal runoff and nutrient removal. In general, annual nutrient and organic matter runoff in Estonian rural catchments is characterized by high losses in spring and autumn and low losses in summer. For instance, the most significant change in mean-monthly nutrient-runoff pattern is that of total-N. In all subcatchments except the upper-course, a significant decrease in nitrogen runoff can be observed. At the same time

U8 . Mander et al. / Ecological Engineering 14 (2000) 405–417 Table 2 Linear regression between the change of share of arable land (x) in subcatchments of the Porijo˜gi River and runoff (y) of organic matter (BOD5), total inorganic nitrogen (TIN; sum of NH4-N, NO2-N and NO3-N), ammonia nitrogen (NH4-N), total phosphorus (Ptot) and sulfates (SO4; all in kg ha−1 year−1) Parameters

Determination coefficient (R 2)

Porijogi entire catchment BOD5 0.90* TIN 0.66* NH4-N 0.008 Ptot 0.88* SO4 0.36

Regression formula

y=0.708x−13.67 y=0.132x+0.93 y= 0.048x−0.90

Porijogi upper course BOD5 0.023 TIN 0.044 NH4-N 0.30 Ptot 0.014 SO4 0.006 Sipe ditch BOD5 TIN NH4-N

0.55** 0.74* 0.45**

y= 0.097x−1.24 y= 0.091x−1.25 y=0.0.0014x

Ptot SO4

0.83* 0.83*

+ 0.009 y= 0.0.014x−0.22 y= 1.451x−21.29

Va¨nda ditch BOD5 TIN NH4-N Ptot SO4

0.42** 0.56** 0.31 0.71* 0.55**

y= 0.468x+0.23 y=0.207x+2.44 y = 0.084x−0.40 y= 1.380x+37.73

* PB0.001; ** PB0.01.

changes in monthly runoff of phosphorus were insignificant (Mander et al., 1998). Mean annual losses of total-P indicate a decreasing trend at the beginning of the study period. In 1987 in the entire Porijo˜gi catchment, the average annual phosphorus losses were very high (0.32 kg P ha − 1 year − 1) but still significantly less than recorded in highly loaded areas (1.6 – 36 kg P ha − 1 year − 1; Miller 1979; Kronvang et al. 1993). The trends in the Porijo˜gi River, however, do not seem to coincide with those of larger rivers in eastern Europe. Time-series analysis of nutrient loads carried on the Daugava River in Latvia and

413

the Oder and Vistula Rivers in Poland showed that the flow-normalized riverine loads remained practically constant from 1989 to 1995 and that most of the interannual variation could be attributed to natural variation in runoff (Grimvall et al., 1999). Therefore, further investigations are needed to get an adequate picture on the relationship between the changing fertilization load and riverine transport on different hierarchical levels of drainage basins. Another significant question is what the nutrient transport response would be if the management intensity in the catchment were to increase again. In the agricultural subcatchments of Sipe and Va¨nda, the slight increase in arable lands (from 14.7 to 19.1% in 1995–1997 in Sipe and from 11.2 to 24.65% in 1994–1997 in Va¨nda; Table 1) were not reflected in nutrient runoff (Fig. 4). Nevertheless, further investigations should clarify this relationship. We suggest that the increase in agricultural activities, especially in reconstruction of drainage systems, would require ecotechnological measures to avoid the potential diffuse pollution. Among these measures are the establishment of riparian buffer zones and buffer strips and the use of several seminatural and constructed wetland

3.4. Importance of ecotechnological measures The positive impact of riparian buffer zones is clear when comparing the nutrient losses from subcatchments in the late 1980s before the significant land use change started. For instance, in 1987, runoff of both organic matter (BOD5), TIN, NH4-N, and total-P from the Sipe subcatchment was respectively four, five, nine, and six times lower than from the Va¨nda subcatchment (Fig. 4). Both sub-basins had the same percentage of arable land and comparable fertilization intensity; however, contrary to the Sipe subcatchment, where the area of riparian buffer strips and buffer zones was 7.8 and 420 ha, respectively, the Va¨nda subcatchment had only 0.2 and 56 ha buffering areas (Mander et al., 1997). In 1990, additional buffer zones and riparian buffer strips were planned to control the nonpoint pollution from both subcatchments (Table 3; Mander et al., 1997).

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In 1990 and 1991, 0.1 and 0.2 ha gray alder stands were planted in the Sipe and Va¨nda subcatchments, respectively. A further decrease in agriculture led to cancellation of the planned work. However, as result of the extensification process, during 1990 – 1997, the area of arable lands decreased 224.1 and 61.6 ha, respectively. In the Sipe subcatchment it was about ten times more than planned to be converted into the buffer zones, whereas in the Va¨nda sub-basin it makes only about a half (Table 3). In fact, this extensification was not a planned process and not only riparian territories were set aside. However, the measured decrease in nitrogen runoff is many times higher than the estimated effect of planned buffer zones and buffer strips. It seems that the nitrogen losses are related more to the entire fertilization load than riparian buffers. In terms of Table 3 Planned and measured efficiency of planned in 1990 buffer zones and buffer strips in two sub-catchments of the Porijo˜gi River Estonia (see Mander et al., 1997) Sub-catchment Planned buffer zones (ha)a Decrease in arable lands 1990–1997 (ha)

Sipe 22

Va¨nda 112

224.1

61.6

Planned buffer strips (ha)b

0.8

1.6

Constructed buffer strips (ha)

0.1

0.2 −1

Estimated efficiency of planned buffer zones (kg ha year−1) N 0.42 13.0 P 0.039 0.14 Estimated efficiency of planned buffer strips (kg ha−1 year−1) N 0.37 6.1 P 0.058 0.11 Total estimated efficiency (kg ha−1 year−1) N 0.79 P 0.097

19.1 0.25

Measured decrease in runoff 1990–1997 (kg ha−1 year−1) N 7.7 29.9 P 0.08 0.18 a

Arable land planned to be converted into less intensively used grasslands. b Forest or bush directly on the stream bank.

phosphorus this relationship is more complex but in the areas with low potential erosion the fertilization level determines the losses. It should be noted that the methodology for estimation the efficiency of planned buffer zones and buffer strips has been worked out for a determined level of fertilization (i.e. in the end of 1980s in the Porijo˜gi catchment: 150 kg N ha − 1 year − 1 and 60 kg P ha − 1 year − 1; Mander et al., 1997). In addition, climatic and hydrological factors (e.g. increasing water discharge) were not considered in this method. However, if the agricultural activity increases, the buffering ecosystems will probably regain their importance. In addition to buffer zones and buffer strips, several wetlands in the catchment will effectively control the nutrient fluxes (Fleischer and Hamrin, 1988; Vought and Lacoursie`re, 1998). Constructed wetlands have been used mostly for purification of wastewater, and to improve water quality in streams, rather than for nonpoint pollution purification (Kadlec and Knight, 1995). However, certain prototypes of constructed wetlands (e.g. macrophyte ponds and shallow wetlands, artificially flooded meadows, root filters, and streamside wetlands) can be effectively used for treatment of polluted waters from agricultural fields (see Gustafson et al., 1999). They can be located in gullies, next to places of intensive fertilization or manure storage, for purification of rainwater from manufacturing areas, roads, parking places, etc. According to Swedish research, the optimal area of various wetlands to guarantee the satisfactory nutrient retention in agricultural catchments is up to 2–3% (Vought and Lacoursie`re, 1998). However, the location of these wetlands in the catchment is a key question. Only those located downhill and next to critical source areas (Pionke et al., 1999) or in the riparian zone can provide a significantly efficient nutrient retention. For instance, an 8.5 ha wetland forest in the upper course of the Va¨nda subcatchment, which covers 3.8% of the area, does not play any important buffering role for downstream water quality. At the same time, the riparian fens and wet meadows along the Sipe stream (0.8% of the subcatchment area, which buffer 32% of the total

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stream bank length) effectively control both the water discharge and quality. In the case of increasing agricultural activities in the catchment the following principles are recommended: 1. All existing and riparian buffer strips should be maintained and regularly harvested; 2. As an alternative to riparian buffer strips small, 0.5–1.0-m deep sedimentation ponds with aquatic macrophytes should be constructed in the gullies and other depressions within the catchment area; 3. In riparian zones, land use intensification (i.e. conversion of grasslands into arable lands) should be avoided or compensated by wider or additional buffer strips (for terms ‘buffer zone’ and ‘buffer strip’ the methodology to determine see Mander et al., 1997); 4. The dimensions of new buffer strips and sedimentation ponds the relevant methodology has been worked in Estonia (Riigi Teataja, 1997, for buffer strips see also Mander et al., 1997). All these principles are stated in the Regulation No. 64 from December 24, 1996, constituted by the Ministry of the Environment of the Republic of Estonia (Riigi Teataja, 1997).

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During the period 1987–1997, the runoff of total-N, total-P, SO4 and organic material (BOD5) in the Porijo˜gi River catchment (southern Estonia) decreased from 25.9 to 5.1, from 0.32 to 0.13, from 78 to 48, and from 7.4 to 3.5 kg ha − 1 year − 1, respectively. The most significant decreases by factors of 4–20 were found in agricultural subcatchments while in the forested upper course catchment the changes were insignificant. In the case of increasing agricultural activities in the catchment several ecotechnological measures (e.g. riparian buffer zones and buffer strips, constructed wetlands) are recommended to control nutrient flows. This is also stated in regulations on landscape ecological principles and water protection in land reclamation areas by the Ministry of the Environment of the Republic of Estonia Acknowledgements This study was supported by grants from the Ministry of Agriculture of the Republic of Estonia (1987–1992) and by the Estonian Science Foundation grants No. 187 (1993–1995) and 2471 (1996–1998). We also acknowledge the Estonian Meteorology and Hydrology Institute for permitting us to use hydrological data.

4. Conclusions References Two factors, change in land-use pattern and climate fluctuations, play the most essential roles in changing the water budget and nutrient flows in the Porijo˜gi River catchment. Milder winters (an increase of air temperature in February from − 7.9 to − 5.5°C during 1950 – 1997) and a change in the precipitation pattern have influenced the mean annual water discharge. This has resulted in more intensive material flow during colder seasons and decreased water runoff in summer. The main trend in land-use changes is a significant increase in abandoned lands (from 1.7% in 1987 to 10.5% in 1997), and a decrease in arable lands (from 41.8 to 23.9%). Coupled with decreasing fertilization, this is caused a significant decrease in nutrient and organic matter release from fields.

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